In recent year, combined-cycle power generation currently becomes the major steam in thermal power plants, in order to improve thermal efficiency of the plants. A combined-cycle power plant is a power plant in which a heat recovery steam generator is combined with a gas turbine and a steam turbine. A combustor provides the gas turbine with combustion gas of high temperature and high pressure. The gas turbine is revolved by expansion of the combustion gas so that a generator is rotated. Thereafter, exhaust gas is introduced into the heat recovery steam generator. In the heat recovery steam generator, steam is generated by utilizing thermal energy of exhaust gas. Steam is supplied to the turbine, and the generator is rotated by the steam turbine in collaboration with the gas turbine.
In general, a conventional heat recovery steam generator is a boiler which generates steam corresponding to heat of exhaust gas discharged from the gas turbine in order to supply steam to the steam turbine. A heat recovery steam generator provided with an auxiliary burner, which is capable of heating up exhaust gas, has been prevails in recent year. This is because decrease in the output of the gas turbine in the summer season results in reduction in an exhaust gas amount, so that it is necessary to compensate decrease in amount of steam generated by the heat recovery steam generator, and because steam should be also supplied to a cogeneration plant, a distilling plant and so on, in addition to the steam turbine.
In a newly developed heat recovery steam generator, an auxiliary burner shows a tendency to have a large size, with a view to increasing a steam supply amount. As a result of it, as exhaust gas temperature in the heat recovery steam generator rises, auxiliary burners are installed on a plurality of locations for the purpose of preventing the generator constituent parts from deteriorating the durability and reliability (see JP2001-116208A, for example).